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991.
Rational harmonic mode locking takes place in an actively mode-locked fiber laser when the modulation frequency fm=(n+1/p)fc, where n and p are both integers and fc is the inverse of the cavity round-trip time, the 22nd order of rational harmonic mode locking has been observed when fm ≈1 GHz. An optical pulse train with a repetition rate of 40 GHz has been obtained using a modulation frequency fm=10 GHz. The theory of rational harmonic mode locking has also been developed. The stability of the mode-locked pulses is improved considerably when a semiconductor optical amplifier is incorporated into the fiber laser cavity. The supermode noise in the RF spectrum of a mode-locked laser is removed for a certain range of current in the semiconductor optical amplifier 相似文献
992.
Z. Huang P. Kumar I. Dutta R. Sidhu M. Renavikar R. Mahajan 《Journal of Electronic Materials》2014,43(1):88-95
A fracture mechanism map (FMM) is a powerful tool which correlates the fracture behavior of a material to its microstructural characteristics in an explicit and convenient way. In the FMM for solder joints, an effective thickness of the interfacial intermetallic compound (IMC) layer (t eff) and the solder yield strength (σ ys,eff) are used as abscissa and ordinate axes, respectively, as these two predominantly affect the fracture behavior of solder joints. Earlier, a definition of t eff, based on the uniform thickness of IMC (t u) and the average height of the IMC scallops (t s), was proposed and shown to aptly explain the fracture behavior of solder joints on Cu. This paper presents a more general definition of t eff that is more widely applicable to a range of metallizations, including Cu and electroless nickel immersion gold (ENIG). Using this new definition of t eff, mode I FMM for SAC387/Cu joints has been updated and its validity was confirmed. A preliminary FMM for SAC387/Cu joints with ENIG metallization is also presented. 相似文献
993.
Multiferroics: Multiferroic Clusters: A New Perspective for Relaxor‐Type Room‐Temperature Multiferroics (Adv. Funct. Mater. 13/2016)
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994.
Subir Panja Salman Ahsan Tanay Pal Simon Kolb Wajid Ali Sulekha Sharma Chandan Das Jagrit Grover Arnab Dutta Daniel B. Werz Amit Paul Debabrata Maiti 《Chemical science》2022,13(32):9432
The Fujiwara–Moritani reaction is a powerful tool for the olefination of arenes by Pd-catalysed C–H activation. However, the need for superstoichiometric amounts of toxic chemical oxidants makes the reaction unattractive from an environmental and atom-economical view. Herein, we report the first non-directed and regioselective olefination of simple arenes via an electrooxidative Fujiwara–Moritani reaction. The versatility of this operator-friendly approach was demonstrated by a broad substrate scope which includes arenes, heteroarenes and a variety of olefins. Electroanalytical studies suggest the involvement of a Pd(ii)/Pd(iv) catalytic cycle via a Pd(iii) intermediate.The Fujiwara–Moritani reaction using electric current is a powerful tool for the olefination of arenes by Pd-catalysed C–H activation.Transition metal-catalysed C–H functionalisation reactions have increasingly gained importance over the last few decades since they allow direct and rapid installation of functionality. Regardless of the undeniable synthetic value of such transformations, the need for superstoichiometric quantities of expensive and hazardous oxidants (e.g., silver and copper salts) remains a major drawback from a sustainable chemistry perspective.1,2 Additionally, chemical oxidants often lead to the formation of by-products, hindering purification and decreasing atom economy. Nevertheless, very few reports were also reported in the literature wherein mild oxidant such as molecular oxygen can also serve as the oxidising agent.2j To make chemical processes and transformations intrinsically sustainable, organic chemists re-discovered synthetic electrochemistry as an environmentally friendly approach.3–6 In the domain of synthetic electrochemistry, the Lei group achieved a significant milestone and installed C–C bonds through a different cross-coupling strategy.1k,2f–h Electroorganic synthesis utilizes electric current to realize redox processes and thereby avoids the use of dangerous, expensive, and polluting chemical oxidising or reducing agents. Precise control of electrochemical reaction parameters often leads to commendable reactivity and chemoselectivity and hence to an improved atom economy. In addition, electrochemical processes fulfil the expectations of sustainability since electricity can be generated from renewable energy sources, such as wind, sunlight or biomass. Recent efforts in the field of electrochemical C–H activation resulted in significant progress towards efficient C–C and C–heteroatom bond formations.7–10 Hence, the utilization of electric current as an alternative oxidant in Pd-catalysed C–H functionalisations is emerging as an attractive alternative to stoichiometric reagents.11–13The Fujiwara–Moritani reaction is one of the earliest known examples of Pd-catalysed oxidative C–H functionalisations for C–C bond formation.14 This extraordinary C(sp2)-H alkenylation reaction avoids the use of prefunctionalised starting materials; however, it suffers from the drawbacks of regioselectivity, reactivity and use of excess arenes.15 Since its development, a number of modified strategies have been reported by different research groups to address the issue of reactivity and selectivity.16–21 In recent times, the ligand assisted oxidative C–H alkenylation of arenes without directing substituents has been established as one of the major strategies to overcome the reactivity issue and to elaborate the substrate scope.However, regioselectivity for most of the sterically and electronically unbiased arenes is still not up to the mark. The most recent studies on the non-directed oxidative C–H olefination of arenes were reported independently by Yu and van Gemmeren (Scheme 1). The Yu group employed electron-deficient 2-pyridone as an X-type ligand for the olefination of both electron-rich and electron-poor arenes including heteroarenes as the limiting reagent (Scheme 1a).18 The pyridone ligand improves the selectivity in a non-directed approach as compared to the directed C–H olefination reaction by enhancing the influence of steric effects. On the other hand, the van Gemmeren group utilizes two complementary ligands N-Ac–Gly–OH and a 6-methylpyridine derivative in a 1 : 1 ratio to accomplish the non-directed olefination reaction of arenes (Scheme 1b).20 Despite the indisputable advances made by these research groups in the area of non-directed oxidative C–H olefination of arenes, the use of superstoichiometric amounts of toxic and waste-generating oxidants (Ag salts) deciphers into a strong call for an environmentally responsive and atom-economic protocol. To address these shortcomings, we recently introduced Pd-photoredox catalysed olefination of non-directed arenes with excellent site selectivity under oxidant free conditions.21Open in a separate windowScheme 1Recent approaches to sustainable C–H alkenylation reactions.In 2007, Jutand reported the directing group assisted Pd-electracatalysed ortho-olefination of acetyl protected aniline in a divided cell by utilizing catalytic amounts of benzoquinone as a redox mediator (Scheme 1c).22a A Rh-catalysed ortho-C–H olefination of benzamide was developed through an electrooxidative pathway by the Ackermann group (Scheme 1d).22b Simple arenes that bear no directing groups are cheap, easily available and very desirable starting materials. However, the use of such arenes is significantly more challenging for selective functionalisation as transformations often result in the formation of complex product mixtures. With no report of an electrooxidative Pd-catalysed C(sp2)-H alkenylation of simple arenes present, we wish to present such a variant of the Fujiwara–Moritani reaction (Scheme 1e). The developed method proceeds through a non-directed pathway and is controlled by stereoelectronic factors. This protocol does not require additional chemical oxidizing agents and is executed using an operator-friendly undivided cell setup.To start our study, naphthalene was chosen as a challenging substrate because of its ability to form α- and β-products. We examined various reaction conditions for the desired Pd-catalysed electrooxidative C–H alkenylation in a simple undivided cell setup (†) with n-butyl acrylate as the coupling partner. After rigorous optimisation, we found that naphthalene reacts with n-butyl acrylate in dichloroethane (DCE) in the presence of Pd(OAc)2 (10 mol%), ligand L1 (20 mol%), and the electrolyte tetra-n-butylammonium hexafluorophosphate (TBAPF6, 0.5 equiv.) while employing a graphite felt anode and a platinum cathode maintaining constant current electrolytic conditions (j = 2.5 mA cm−2, Entry Alteration from standard conditions Yield of 1b (%) Selectivity (β : α) 1 None 70 >25 : 1 2 Co(OAc)2·4H2O instead of Pd(OAc)2 9 1 : 1 3 [Ru(p-cymene)Cl2]2 instead of Pd(OAc)2 NR 4 Pd(OAc)2·(5 mol%) 51 >25 : 1 5 Pd(OAc)2·(20 mol%) 71 >25 : 1 6 L2 instead of L1 45 8 : 1 7 L3 instead of L1 59 20 : 1 8 L4 instead of L1 19 5 : 1 9 L5 instead of L1 8 1 : 1 10 Benzoquinone (10 mol%) 68 >25 : 1 11 PivOH (1.0 equiv.) 61 20 : 1 12 Ni foam instead of Pt 64 >25 : 1 13 GF instead of Pt 49 15 : 1 14 Steel instead of Pt 31 13 : 1 15 6 mA cm−2 instead of 2.5 mA cm−2 27 11 : 1 16 24 h reaction time 47 20 : 1 17 12 h reaction time 56 21 : 1 18 No electricity NR — 19 No Pd(OAc)2 NR —